Method of preparing ferroelectric powders and ceramics

ABSTRACT

The present invention relates methods of making a ferroelectric powder and ceramics having the steps of mixing a polymer and a metal oxide precursor mixture within a mill, milling the polymer/precursor mixture, drying the mixture, burning the dried solid, grinding the solid, and calcinating the solid to a ferroelectric powder. The present invention in particular introduces a polymeric species with a metal oxide precursor at the milling stage. The production of ceramics further involves compressing the ferroelectric powder and sintering the compression.

BACKGROUND

The development of electronic devices such as sensors, actuators, and capacitors requires materials with desired crystal structure and excellent electromechanical properties, as well as a reliable processing route. Low cost and low temperature processing of the raw materials are necessary for large-scale applications.

In recent years, perovskite oxide solid-solution based on Pb-containing and Bi-containing systems have attracted great attention, due to the spontaneous polarization in these materials, which leads to excellent ferroelectric, piezoelectric, and pyroelectric properties. In the synthesis of ceramics containing the perovskite phase, such synthesized ceramics suitable for producing electronic devices, one of the challenges is to suppress, and in fact avoid altogether, the formation of the undesirable pyrochlore phase. The pyrochlore phase, if present in ceramics, is deterimential to the electrical properties of the ceramics and the eventually manufactured electronic devices.

However, many ferroelectric materials can be synthesized by a conventional solid state reaction method, e.g., all the raw materials (metal oxides and metal carbonates) are mixed together for a ball-milling process and calcined at 850° C.-950° C. to form perovskite phase. PZT can be prepared through this method, but preparation of PMN-PT and other PB-containing relaxor type materials with this process is failed due to the preferable formation of the pyrochlore phase. In some cases, the pyrochlore phase can be transformed into the perovskite phase by increasing the calcination temperature to well above 900° C. In other cases, specifically where relaxor-type ferroelectric materials are used, once the pyrochlore phase has formed, it is difficult to remove, even with the use of high heat.

In attempting to solve this problem, a two-step calcination technique called the columbite precursor method was developed by S. L. Swartz and T. R. Shrout (Materials Research Bulletin, Vol. 17, pp. 1245-1250, 1982). In the prior art, PMN-PT powders can be prepared with a two-step columbite method. In the first step, MgO and Nb₂O₅ are mixed and ball-milled, followed by a calcinations process at above 1000° C. to form a columbite phase of MgNb₂O₆. In the second step, the columbite MgNb₂O₆ is mixed with PbO and TiO₂, ball-milled and calcined at about 900° C. to form perovskite phase. In this method, the Nb₂O₅ and PbO are isolated to avoid the formation of pyrochlore phases such as Pb₃Nb₂O₇ or Pb₃Nb₄O₁₃. However, this two-step process increases the overall cost of manufacturing ferroelectric powders and ceramics as it increases the time and material usage.

U.S. Pat. No. 3,330,697 to Pechini teaches a polymer species added to a solution to form polymer-cation complexes which aides the crystallization. However, the liquid precursor methods are not suitable for mass production in the thick film and ceramics synthesis field.

It is an object of the present system to overcome the disadvantages and problems in the prior art.

DESCRIPTION

The present system proposes a polymer-aided solid state reaction method to eliminate or reduce the amount of pyrochlore phase in a relaxor-type ferroelectric materials.

The present system also proposes a method to modify the conventional solid state reaction process (one-time mixing and calcinations) by introducing a polymer to the process and successfully synthesized ferroelectric powders and ceramics with the desired perovskite structure.

The present system further proposes a production of ferroelectric powders and ceramics at reduced temperatures in comparison with the prior art.

These and other features, aspects, and advantages of the apparatus and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings where:

FIG. 1 shows the method of producing powders in accordance with the prior art conventional solid state reaction process.

FIG. 2 shows a method of producing ferroelectric powders in accordance with the present method.

FIG. 3 shows the method of producing ceramics in accordance with the present method.

FIG. 4 (A) and (B) shows a comparison of a ferroelectric powder produced in accordance with the present invention and a ferroelectric powder produced in accordance with the prior art.

FIG. 5 is a scanning electron microscope image of the surface of a ferroelectric ceramic made in accordance with the present method (A)-(C), in comparison with the surface of a ceramic made under the prior art methods.

FIG. 6 shows the temperature dependence of dielectric constant (εr) and loss (tan δ) for the PMN-PT ceramics synthesized by the present method.

FIG. 7 shows the ferroelectric hysteresis loops of the PMN-PT ceramics made in accordance with the present method.

The following description of certain exemplary embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Throughout this description, the term “perovskite phase” and “perovskite material” refer to a ferroelectric material provided with a perovskite structure including layered perovskite structure or a perovskite phase;

the term “polymer” refers to organic compounds or materials comprising monomer unit structure, including a monomer and oligomer; and

the term “pyrochlore phase” refers to a general crystal structure of the type A₂B₂O₆, A₂B₂O₇, A₃B₂O₈, and A₃B₄O₁₃ where the A and B species are generally rare-earth or transition metal species.

Now, specifically to FIGS. 1-7,

FIG. 1 is an example of the method of making ferroelectric powders in the prior art. Prior art for the method of producing ferroelectric powders for attempting to avoid the pyrochlore phase include the Columbite Precursor Process, which is a two-phase method. The first phase involves mixing of a precursor powder. The precursor is then ball-milled, followed by calcinations at above 1000° C. to form a columbite phase compound. The second phase involves the mixing of the columbite phase compound with a lead compound and a titanium compound. This prepared compound is ball-milled and calcined at above 900° C. to form a perovskite phase compound (resultant powder). The drawbacks of such a system include increase in cost and time to produce industry acceptable powders.

FIG. 2 is a method of making ferroelectric powders in accordance with the present invention, whereby ferroelectric powders having predominantly perovskite crystal structure are formed by introducing a polymeric species to the milling process. The present method comprises the steps of mixing 205 a polymer 201 with a metal precursor mixture 203 within a mill, milling 207 the polymer/precursor mixture, drying 209 the mixture burning 211 the resultant solid, grinding 213 the solid, and calcinating 215 the solid to the resultant powder 217.

The metal precursor mixture 203 may be comprised of metal oxide raw materials of, for example, compounds in the II state, such as BaO, PbO, MgO, NiO, SrO, ZnO, and CoO compounds in the III state, such as La₂O₃, NiO₃, Bi₂O₃, and CoCO₃ compounds in the IV state, such as TiO₂ and ZrO₂, and compounds in the V state, such as Nb₂O₅ and Ta₂O₅. The mixture may comprise two or more metal compounds, the compounds capable of coming from one or more states. In one example, the metal precursor is made of a mixture of PbO, MgO, Nb₂O₅, and TiO₂. In another example, for Pb(A²⁺ _(1/3)Nb_(2/3))O₃ system, the precursor of A²⁺ can be a combination of ZnO and NiO, ZnO and MgO, or NiO and MgO. The metals for inclusion in the precursor mixture may be selected based upon the desired resultant powder, for example if the desired resultant powder is Pb(Mg_(1/3)Nb_(2/3))O₃ (PMN), the metal precursor mixture can contain PbO, MgO, and Nb₂O₅. Further, metals may be used to substitute for one another in leading to the desired powder. For example, if Zn or Ni, instead of Mg, is chosen for Pb(A²⁺ _(1/3)Nb_(2/3)) O₃ system, the compound ZnO or NiO is employed. Preparation of the resultant powder and ceramics would then follow that of MgO. In another example, when Ta, instead of Nb, is chosen for the system, a Ta₂O₅ compound is employed. Preparation of the resultant powder and ceramics would then follow that of Nb₂O₅.

In another embodiment, metal carbonates may be used to form the precursor mixture. Examples include COCO₃, BaCO₃, LaCO₃, LiCO₃, MgCO₃, MnCO₃, NiCO₃, and SrCO₃.

Additional adjuvants may be added to the precursor mixture, including excess amounts of lead. Excess lead can be added to compensate for volatilization of lead during heat treatment of the metals. Excess amounts can range from 3 to 7 mol %. Further adjuvants can include dopant atoms, which affect the ferroelectricity of the resultant powder. Suitable dopants include Nb, Fe, La, and Al alkoxide compounds.

A polymer 201 suitable for mixing with the precursor mixture can be selected based upon its species, molecular weight, and concentration. It is to be noted that the elimination of the pyrochlore phases in the derived powders or ceramics of the instant invention are affected by the type of polymer species employed, the molecular weight of the polymer, and the concentration of the polymer species in the metal oxide precursor mixture. The role of the polymer in the present invention is to stabilize the perovskite during heating process and suppress the pyrochlore phase.

Suitable polymer species or monomers have a molecular weight below 10,000. Suitable polymer species can include hydrophilic, hygroscopic, and thermally stable polymers such as polyethylene glycol, whose formula corresponds to

HO—(CH₂—CH₂—O)_(n)—H

In one embodiment, the polyethylene glycol (PEG) with an average molecular weight equal to or less than 10,000 are suitable for use herein. In another embodiment, PEG with a molecular weight of 200 is used herein. In still another embodiment, PEG can have a molecular weight of 2000.

Other suitable polymers for use herein include polyethylene glycol monoethyl ethers, copolymers such as polyethylene glycol/polypropylene glycol copolymers, polypropylene glycol, polyvinyl pyrolidone, and polyvinyl alcohol. In all instances, the average molecular weight should be less than 10,000. The amount of polymer used in the solution with a metal oxide precursor mixture can be within a ratio of 0.5 to 1.5 to the mol amount of Pb in the case where a Pb-oxide compound is used in the precursor mixture, or a ratio of 0.5 to 1.5 to the mol amount of Bi in the case where a Bi-oxide compound is used in the precursor mixture. In one embodiment, the polymer is 1:1 equivalent in terms of mol amount to Pb or Bi.

Mixing 205 of the polymer species and precursor mixture 203 occurs during the milling 207 stage. Milling 207 can include ball milling, vibrating ball milling, rod milling, hammer milling, and the like. The milling media in the ball mill may be ceramic balls, metal balls, steel rods, and the like. Preferably, ceramic balls are used to avoid contamination. The ceramic milling media may be, for example, alumina, zirconia, silica, magnesia, and the like. The milling time may be from a few minutes to a few days, generally between a few hours to 24 hours. In one embodiment, the milling of the polymer/precursor mixture can occur for 24 hours.

In mixing within the mill, the precursor mixture 203 can be added first to the mill, followed by the addition of the polymer 201. In another embodiment, polymer 201 is added first, followed by the precursor mixture 203. In yet another embodiment, the polymer 201 and precursor mixture 203 are added together outside of the mill, then added to the mill for mixing 205 and milling 207.

Following milling 207, the milled polymer/precursor mixture can be dried 209 between a range of about 95° C. and about 115° C. for a period of between 1.5 and 3 hours. This step is undertaken to remove any liquid medium. The dried mixture should be devoid of any solvent. In another embodiment, during drying, a solvent recovery system is employed to recycle the solvent. In one embodiment, the mixture is dried at 100° C. for 2 hours.

Following drying 209, the dried mixture is burned 211 at a temperature of between 250° C. and 600° C. for a period of 3 to 4.5 hours. Burning 211 brings about the decomposition of the polymer accompanied with the early stage (or trigger) of crystallization. In one embodiment, the mixture is burned at 400° C. for 4 hours.

The mixture can then be ground 213 by methods known in the art, including mechanical grinding, crushing, rolling, and the like. Grinding also includes the steps of pressing the sample into pellets.

The mixture is then calcinated 215 at a temperature of between about 800° C. and 1000° C. for between 1.5 and 5 hours. Calcination 215 can occur inside a furnance, or other suitable means. In a preferred embodiment, calcination occurs between 850° C. and 900° C. for a period of between 2 and 2.5 hours. Advantageously, the present invention utilizes reduced temperatures, when compared with the prior art, during calcination to form the perovskite phase.

Following calcinations 215, a resultant powder 217 is obtained. As stated previously, the desired resultant powder 217 is created based upon the metals added to the metal oxide precursor 203. Suitable resultant powders made in accordance with the instant invention include ferroelectric materials. Ferroelectric materials are a class of materials that can be thought of as having electrical properties somewhat analogous to the magnetic properties of ferromagnetic materials. Relaxor-type ferroelectric materials are ferroelectric materials that have a large electro-mechanical coupling factor and piezoelectric coefficient. Exhibited relaxor-type ferroelectric behavior includes high dielectric constant and low dielectric loss (at room temperature).

Ferroelectric powders produced according to the instant method include Pb or Bi-containing materials, including PbTiO₃ (PT), Pb(Zr, Ti)O₃ (PZT), Bi₄Ti₃O₁₂, and ABi₄Ti₄O₁₅ (where A=Sr, Ba, and Ca). Relaxor-type ferroelectrics made using the present method include those in which the pyrochlore phase is difficult to remove, such as Pb (Mg_(1/3)Nb_(2/3))—PbTiO₃ (PMN-PT), Pb(Ni_(1/3)Nb_(2/3))—PbTiO₃ (PNN-PT), Pb (CO_(1/3)Nb_(2/3))O₃, Pb(Zn_(1/3)Nb_(2/3))—PbTiO₃ (PZN-PT), and (Bi_(1/2)Na_(1/2))TiO₃.

In one embodiment, the resultant ferroelectric powders are those which have a morphotrophic phase boundary (MPB) where rhomobohedral and tetragonal phases coexist. Around the MPB, the percentage of pyrochlore phase can be effectively reduced in the resultant powders and ceramics of the materials. In one embodiment, the MPB compositions of (1-x)PMN-xPT, (1-x)PNN-xPT, and (1-x)PZN-xPT are within x ranging from about 0.32 to about 0.35, from about 0.30 to about 0.40, and from about 0.08 to about 0.10, respectively.

FIG. 3 teaches a method of synthesizing ceramics in accordance with the present invention, utilizing the resultant powders made in accordance with the instant method.

The resultant powders 301 are firstly compressed 303 with the assistance of a binder. Binders suitable for used herein include organic binders, polymers, waxes, gums, and the like. An example of a suitable organic binder would be polyvinyl alcohol. Inorganic binders may also be used, including clays, silicate, phosphates, and the like.

Compression can occur by pressing in die method, such as single punch, single punch in a floating container, punch with two counteracting forces, omnidirectional pressure method, or cold isostatic pressing method. In another embodiment, compression may be combined with sintering to form one step process. An example would be hot isostatic pressing (HIP).

In sintering 305, the pellets are exposed to a high temperature of about 900 to about 1100° C. In an alternative embodiment, the pellets are first exposed to low heat to burn off the binder, followed by exposure to high heat. Sintering may occur between 1.5 and 3 hours. In one embodiment, sintering occurs for about 2 hours. In comparison with the prior art method of making ceramics from ferroelectric powders, in particular reflexor-type ferroelectrics, reduced temperatures can be used during the sintering step. The use of reduced temperatures is especially meaningful when the ceramics are integrated with the metallic electrode such as silver-palladium (Ag—Pd) or platinum (Pt) and undergone a cofiring process below the melting temperature of the electrode.

It is believed that, through the use of the polymer in aiding the development of the ferroelectric powder, the present invention weakens the directional effect of the lone pair of electrons in Pb²⁺ or Bi³⁺, thus eliminating the pyrochlore phase in Pb-containing and Bi-containing materials.

FIGS. 4-7 relate to the following example exhibiting the present invention,

EXAMPLE

In this example of the present invention, the preparation of (1-x)PMN-xPT powders and ceramics with x=0.35 are described.

The oxide raw materials of PbO, MgO, Nb2O5, and TiO2 are mixed with polyethylene glycol with a molecular weight of 200 (PEG200), followed by a ball milling process in ethanol. An excess of PbO of 5% was added to precursor in order to compensate the lead loss during calcinations and sintering. The amount of PEG is equal to that of Pb in mol. After ball milling for 12 hours, the PEG/precursor mixture is dried at 100° C. to evaporate ethanol, and burned at 400° C. for 4 hours for the decomposition of PEG. The dried mixture is then ground and pressed into pellets with PVA as binder, which is then calcined in the furnace at 850° C. for 2 hours. For comparison, the oxide precursor without PEG is also ball milled and calcined at 850° C. for 2 hours.

FIG. 4 at (A) shows that x-ray diffraction (XRD) results indicated that the powders prepared from the PEG200-modified oxide precursor shows perovskite phase, without pyrochlore phase being detected. The perovskite phase is stable in the temperature range of about 850° C. to about 1000° C.

FIG. 4 at (B), in contrast, shows for the oxide precursor without PEG added, a great deal of pyrochlore formed which coexisted with the perovskite phase even when the sample is calcined at 1000° C. This result is consistent with the previous reports on the synthesis of the PMN-PT powders prepared with the direct solid state reaction method, in which a large amount of pyrochlore phase was formed and was hard to remove.

As is known in the art, the steps of mixing, milling, drying, burning, grinding, calcinating, and sintering may be adjusted to meet the needs of scale-up that would be required to produce manufacturing-level amounts of ferroelectric powders and ceramics, such manufacturing-level amounts being suitable for worldwide distribution. Scaling up would be known to one with ordinary skill in the art to the efficiency required to effectively apply the invention.

FIG. 5 at (A)-(C) shows the surface morphology observed by scanning electron microscope (SEM) for ceramics obtained by compressing the powder into pellets with polyvinyl alcohol (PVA) and sintering at a temperature between 900° C. and 1000° C. The surface morphologies are similar at different temperature, with grain size ranges in 1-3 μm.

FIG. 5 at (D) shows surface morphology by SEM for a powder without the polymer modification. The grains are shown to be quite small, below 1 μm, with very large pyrochlore grain distribution.

FIG. 6 shows the temperature dependence of dielectric constant (εr) for the PMN-PT ceramics synthesized in this invention and sintered at 1000° C. The dielectric constant is about 3700 at room temperature, which is comparable with the values usually obtained for PMN-PT ceramics prepared via columbite method. The peak at about 178° C. in the figure indicates the ferroelectric to paraelectric phase transition temperature (T_(c)) upon heating, in agreement with the PMN-PT phase diagram reported previously. A maximum value of the dielectric constant (εmax) of 15700 is attained at the T_(c) at a frequency of 1 kHz.

FIG. 7 shows the ferroelectric hysteresis loops of the PMN-PT ceramics sintered at 1000° C. As the applied electric field increases, the polarization and coercive field increases accordingly. The coercive field and remnant polarization are 8.2 kV/cm and 13.0 μC/cm², respectively, under an applied electric field of 250 kV/cm. The measured d₃₃ value is 218 pC/N. These properties make the material promising for applications in advanced electromechanical devices.

FIG. 8 shows the XRD patters of the 65PMN-35PT powders prepared from precursor oxides modified with different amounts of PEG200 (PEG200:Pb ratio of 1.5:1, 1:1, and 0.5:1). As shown, when the mol ratio of PEG200:Pb in the precursor is 1:1, the undesired pyrochlore phase is undetectable. Even when the mol ratio of PEG200:Pb is above or below a ratio of 1:1, the amount of pyrochlore phase is reduced substantially.

FIG. 9 shows the x-ray diffraction (XRD) patters of 65PMN-35PT powders prepared from the precursor oxides modified with monomolecular ethylene glycol (EG) and with PEG of different molecular weight. As shown, the powder drevied from the precursor modified with PEG200 exhibits almost a complete pervoskite phase. As the molecular weight of PEG increases, so does the appearance of the pyrochlore phase.

Having described embodiments of the present system with reference to the accompanying drawings, it is to be understood that the present system is not limited to the precise embodiments, and that various changes and modifications may be effected therein by one having ordinary skill in the art without departing from the scope or spirit as defined in the appended claims.

In interpreting the appended claims, it should be understood that:

a) the word “comprising” does not exclude the presence of other elements or acts than those listed in the given claim;

b) the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements;

c) any reference signs in the claims do not limit their scope;

d) any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise; and

e) no specific sequence of acts or steps is intended to be required unless specifically indicated. 

1. A method of making a ferroelectric powder, comprising the steps of mixing a polymer with a metal oxide precursor within a mill; milling said polymer/metal oxide precursor mixture; drying said polymer/metal oxide precursor mixture; burning said dried mixture; grinding the burned solid; and calcinating the solid.
 2. The method of claim 1, further comprising the step of adding excess lead in the range of 3 to 7 mol % following the mixing of said polymer and said metal oxide precursor.
 3. The method of claim 1, wherein said polymer has a molecular weight of 10,000 or below.
 4. The method of claim 1, wherein said polymer is selected from the group consisting of polyethylene glycol, polyethylene glycol monoethyl ether, copolymer such as polyethylene glycol/polypropylene glycol copolymers, polypropylene glycol, polyvinyl pyrolidone, and polyvinyl alcohol.
 5. The method of claim 1, wherein said polymer is used in a ratio of 0.5 to 1.5 mol to the mol amount of Pb, in the case where a Pb-oxide compound is used in said precursor mixture, or in a ratio of 0.5 to 1.5 mol to the mol amount of Bi, in the case where a Bi-oxide compound is used in said precursor mixture.
 6. The method of claim 1, wherein said polymer is polyethylene glycol with a molecular weight selected from the group consisting of 200, 2000, or 10,000.
 7. The method of claim 1, wherein said metal oxide precursor mixture is comprised of more than one metal oxide compound.
 8. The method of claim 1, wherein said metal oxide precursor mixture is comprised of one or more of the metal compounds selected from the group consisting of BaO, PbO, MgO, NiO, SrO, ZnO, La₂O₃, NiO₃, Bi₂O₃, TiO₂, ZrO₂, Nb₂O₅, and Ta₂O₅.
 9. The method of claim 1, wherein said metal oxide precursor mixture is made of PbO, MgO, Nb₂O₅, and TiO₂.
 10. The method of claim 1, wherein the mixing of said polymer and said precursor mixture occurs within a mill selected from the group consisting of ball mill, vibrating ball mill, rod mill, or hammer mill.
 11. The method of claim 1, wherein drying said polymer/metal oxide precursor mixture occurs between a range of about 95° C. to 115° C. for a period of between 1.5 and 3 hours.
 12. The method of claim 1, wherein burning said dried mixture occurs at a temperature of between 250° C. and 600° C. for a period of 3 to 4.5 hours.
 13. The method of claim 1, wherein grinding the burned solid comprises utilizing an organic binder during grinding.
 14. The method of claim 1, wherein calcinating said solid occurs at a temperature of between 800° C. and 1000° C. for a period of between 1.5 and 2.5 hours.
 15. A method of making a ceramic using a ferroelectric powder devoid of pyrochlore phase, comprising the steps of obtaining a ferroelectric powder compressing said ferroelectric powder and sintering said ferroelectric powder.
 16. The method of claim 15, wherein said ferroelectric powder is a selected from the group consisting of non-relaxor-type ferroelectric and relaxor-type ferroelectric.
 17. The method of claim 15, wherein said ferroelectric powder is selected from the group consisting of PbTiO₃, PbZrTiO₃, Bi₄Ti₃O₁₂, SrBi₄Ti₄O₁₅, BaBi₄Ti₄O₁₅, CaBi₄Ti₄O₁₅, Pb (Mg_(1/3)Nb_(2/3))—PbTiO₃, Pb (Ni_(1/3)Nb_(2/3))—PbTiO₃, Pb (Zn_(1/3)Nb_(2/3))—PbTiO₃, and Bi_(1/2)Na_(1/2)TiO₃.
 18. The method of claim 15, wherein said ferroelectric powder possess a morphotrophic phase boundary where rhomobohedral and tetragonal phase coexist.
 19. The method of claim 15, wherein compressing said ferroelectric powders occurs with the assistance of a binder selected from the group consisting of organic binders, polymers, waxes, gums, polyvinyl alcohol, clay, silicate, and phosphate.
 20. The method of claim 15, wherein sintering comprises exposing said compressed powder to a temperature between 900° C. and 1100° C. for between 1.5 and 3 hours. 